In the manufacture of paper, plastic film or the like, web breakage occasionally occurs. Web breakage presents the operator with the difficult problem of rethreading the web, especially when the break occurs on continuously running machines, such as papermaking machines, since it is not desirable to shut down the machines completely due to attendant startup difficulties, etc. As a consequence, it is a commonly used technique for the machine operator and his assistants to attempt to manually grasp the web on the fly while the machine continues to operate and direct it into close proximity with the next station, such as a roll nip. It will be appreciated that this is a difficult and dangerous procedure and it is possible for a person attempting to thread the machine in such a manner to become injured, even to the extent of losing fingers or hands. Also, the standard procedure is often quite time-consuming with numerous attempts to thread the web tail often being necessary before success is attained. Until rethread is successfully accomplished, production is lost. Similar problems also occur of course when an end or tail of an unbroken web is threaded initially.
In copending U.S. application Ser. No. 581,306, filed May 27, 1975, a system of relatively inexpensive and simple construction is disclosed which utilizes a phenomenon known as the "Coanda effect" to entrain the end or tail of a moving web of flexible material and direct it to a desired location such as a nip formed between two rotating rolls. The Coanda effect itself has been known for many years, as exemplified by U.S. Pat. No. 2,052,869, issued to Henri Coanda. Briefly, this phenomenon can be described as the tendency of a sheet of fluid, which has been formed, for example, by passing through a slit or similar restricted opening under pressure, to attach itself or cling to and follow a generally curved surface. This creates a zone of reduced pressure in the area of the slit so that relatively large quantities of ambient air or other entrainable material which are in the zone will become entrained and flow with the fluid which has attached itself to the extended lip.
More particularly, apparatus constructed according to the invention of the aforesaid application Ser. No. 581,306 is disposed adjacent to the normal path of movement of a web of flexible material and in proximity with the nip or other desired predetermined location to which the tail of the web is to be directed. The apparatus includes at least one wall element open to the atmosphere along at least a major portion of its length. A restricted opening, preferably in the form of a slit, is formed in the wall element and the restricted opening is in selective communication with a source of pressurized gas such as air. A generally curved flow attachment surface recedes from the restricted opening and leads toward an end of the wall element. The device is actuated by causing passage of the gas under a high velocity from the source of gas through the slit or other suitable restricted opening. Due to the Coanda effect, the gas passing through the restricted opening attaches to the flow attachment surface and is directed in the form of a high velocity gaseous sheet along the wall element toward the end thereof. The high velocity gaseous sheet causes entrainment of ambient air along the wall element. The tail of the web which is to be directed to the predetermined location is directed into the combined gaseous flow along the wall element. Because the velocity of gaseous flow is substantially greater than the speed of the moving web itself, the web tends to straighten out in the direction of the predetermined location and the web is aimed at the location with little or no manual assistance. When the tail enters the nip it is pulled tight and automatically lifts itself from the apparatus into its normal path of movement.
The apparatus according to the above-described invention may take a number of forms. For example, the apparatus may be mounted in a more or less permanent fashion or it may be mobile or even take the form of a handheld operator-manipulated device. Further, the above-described apparatus of application Ser. No. 581,306 may be provided with a plurality of combinations of restricted openings and flow attachment surfaces, i.e. Coanda nozzles.
While the afore-described system disclosed in U.S. patent application Ser. No. 581,306 has operated in an essentially satisfactory manner, some difficulties in operation have been encountered under certain conditions. In particular, under certain conditions the combined flow of entraining gas and entrained air used to transport the web tail can actually interfere with the proper operation of the system. For example, in those tail threading systems employing a plurality of Coanda nozzles sequentially along the tail path, as exemplified by several embodiments illustrated in the aforesaid application Ser. No. 581,306, the combined gaseous flow from the first Coanda nozzle can cause flow interference at the Coanda nozzle positioned upstream therefrom. In an arrangement of the type shown in FIGS. 11-13 of application Ser. No. 581,306 wherein change of web direction is effected at the second Coanda nozzle, this problem becomes particularly acute and the flow from the first Coanda nozzle may carry the web tail too high above the second Coanda nozzle for it to be effective, with the tail having a tendency to float or be thrown off the device unless appropriate flow adjustments are made. The conventional approach to minimizing flow interference is to reduce the pressure and flow rate of the gas exiting from the first Coanda nozzle (used for web pickup) to less than that utilized at the next Coanda nozzle deployed upstream. Often this is undesirable, since more energy is normally required during the web pickup phase of the operation at the initial Coanda nozzle location, where the tail direction is drastically changed and the device often has to handle what is essentially a two-ply tail until the threading is accomplished, than at the second and subsequent Coanda nozzles which are normally used to effect conveyance and actual threading of the web tail. Another approach to elimination of flow interference is to shut off the pressurized air supply at the first Coanda nozzle as soon as the web end or tail reaches the second nozzle. This approach, however, is not usually feasible due to the high speeds at which the tail is conveyed by the system.
Excessive gaseous flow can also cause difficulties even in those arrangements employing a single Coanda nozzle which is utilized to direct a web tail into a roll nip or the like. If the end of the web threading unit is placed into too close a proximity with the nip a "back-flow" problem may result when the excess gaseous flow hits the surfaces of the rollers forming the nip. This can result in the web tail being forced out of the nip by the air flow prior to its being threaded.
The afore-mentioned problems are solved in accordance with the teachings of the present invention by providing a wall element having a web transfer surface open to the atmosphere and having a web entry and a web exit end and employing in combination therewith separator means disposed at the wall element exit end for separating the web from a substantial portion of the combined flow of the entraining gas and entrained ambient air so that excess gaseous flow is not a problem when the web tail reaches its desired downstream directive which may be either a restricted opening-flow attachment surface combination or the ultimate objective which is a threading nip formed by two rollers or the like. Separation of a substantial portion of the gaseous flow and the web is accomplished by providing a diverting wall element connected to the wall element having a web transfer surface and web restraining means for preventing the web from moving along the diverting wall element with a substantial portion of combined gaseous flow diverted therealong. The web restraining means comprises a barrier fixedly positioned at the wall element exit end adapted to be engaged by the web. The barrier has openings therein through which the substantial portion of the combined flow of entraining gas and entrained ambient air may pass freely. The diverting wall element has a generally curved Coanda fluid flow diverting surface for changing the direction of movement of the substantial portion of the combined flow of the entraining gas and the entrained ambient air. The apparatus according to the present invention may take a number of forms. For example, the apparatus may be mounted in a more or less permanent fashion or it may be mobile or even take the form of a handheld operator-manipulated device.
Another aspect of the invention resides in the utilization of a foil element which defines the generally curved fluid flow attachment surface that is angularly positioned relative to the web transfer surface so that the entraining gas impinges against the web transfer surface and the entraining gas traverses a concave path of movement. This feature is utilized when appropriate to place the web path in the vicinity of the slit where maximum suction forces are imparted to the web tail by the gas exiting from the restricted opening and flowing around the foil flow attachment surface. Yet another aspect of the invention resides in the employment of auxiliary nozzle means to blow a web tail off a rotatable dryer can or the like and assist in placement of the web tail into close proximity to the foil. Still another aspect of the invention resides in employing a pair of spaced units in the system, each unit having at least one Coanda nozzle and associated wall element, with the first unit being positioned so as to direct or "pitch" the web end or tail to the second unit which "catches" it and redirects movement of the tail. The units are spaced a substantial distance apart so that the flow of entraining gas associated with the web is substantially dissipated.